Macroporous Rgo Electrode Decorated with Co₃O₄ Nanoparticles for Selective Catalytic Electro-Oxidation of Water

OBJECTIVES The electro-oxidation of H2O to O2 is a promising process to store energy into a green fuel. Its intrinsic high kinetic barrier requires the use of catalysts to lower the applied potential; moreover, it suffers the competition of side-reactions like Cl‒ oxidation to chlorine, an undesirable corrosive by-product. The high concentration of Cl‒ ions in seawater is one of the major obstacles which hinders the direct employment of such widely available resource as the electrolyte for O2 production (Adv. Mater., 2018, 30, 1707261). The benchmark electrocatalysts based on noble metals (Pt, IrO2) are not selective toward Cl‒, and their scarcity prevents their practical large-scale application. Earth-abundant oxides and (oxy)hydroxides of first-row transition metals (FeOOH, NiOOH, Co3O4) are robust and show even higher catalytic performances than Pt/IrO2. Unfortunately they are not selective against Cl‒ oxidation (ACS Catal., 2014, 4, 3701). In this work, we aim to suppress Cl‒ interference on heterogeneous catalysts through a carefully designed electrode architecture. We synthesize a free-standing, conducting, 3D macroporous reduced graphene oxide (rGO) composite with catalytic Co3O4 nanoparticles (NP) selectively deposited just on the internal walls of its closed, not interconnected pores (average diameter 100 µm). The pore walls act as membranes made of multiple stacked rGO nanosheets; the nanochannels between rGO layers have width < 1 nm which previous works have shown to be permeable to water and gases while preventing the diffusion of dissolved ions like Cl‒ (e.g. Nat. Mater., 2017, 16, 1198). Since the catalytic sites are accessible selectively to the substrates able to permeate through the rGO walls, the water electro-oxidation is expected to happen without suffering the competing Cl2 evolution. The most common method for synthesizing 3D rGO porous materials is through self-assembly of graphene oxide (GO) suspensions during hydrothermal reduction. However, this method leads to scaffolds with small open pores with diameters in the micrometer range (e.g. ACS Nano, 2010, 7, 4324). A macroporous closed structure, instead, allows for selectivity together with higher diffusion rates of the permeating species from the outer solution to the reaction sites. For this reason, in our strategy we synthesize the scaffolds starting from emulsions of hexane droplets in water stabilized by GO nanosheets as soft template for the macroporous structure. RESULTS We modify the amphiphilicity of GO by adding different amounts of surfactants to produce so-called high internal phase emulsions (HIPE) stabilized by GO. These are single phase emulsions characterized by a high amount of emulsified phase (Hex:H2O ratio > 0.75), and a long-term stability (several months). Then, we convert the HIPE in the final 3D scaffolds by triggering the self-assembly of GO sheets through a hydrothermal reduction with slow temperature ramp, at the end of which hexane has entirely evaporated due to its low boiling point. Through this simple process we are able to control the architecture of the rGO scaffolds without polymerizations or post-modifications. We can control the pore size by tuning the templating HIPE droplet size, which we show depends on the GO concentration, hexane to water ratio, hexane to volume ratio, and emulsifying method (time and power). Because of its superior structural homogeneity compared to the conventional diluted emulsions previously reported (J. Mater. Chem. A, 2015, 3, 4018), a HIPE brings several advantages to the final material, i.e. better mechanical properties, very high surface area, and lower electrical resistivity (2.6±1.3 kΩcm). This is also the first study which reports the addition of a metal-organic precursor (of Co3+ in this specific case), or metal oxide NP (specifically Co3O4) functionalized with a hydrophobic layer, in the apolar phase during the preparation of the emulsion template, and then exploits the immiscible nature of the apolar and water phases to selectively decorate with catalytic NP only the internal wall of the closed pores. This feature requires the formation of highly stable initial GO HIPE and cannot be achieved by any other strategy previously reported. SIGNIFICANCE OF THE WORK The rGO macroporous materials produced are currently under investigation as self-standing electrocatalysts for selective water oxidation with respect to chlorine evolution. We will then test these electrodes for water decontamination and as anodes in energy storage devices. While in this work we focus on Co3O4, our approach allows the encapsulation of any type of nanocatalyst (metal oxide/metal-based NP) inside the closed rGO macropores, just by changing the hydrophobic precursor. Such materials may exhibit electrocatalytic activity and selectivity towards a variety of substrates that can permeate through the rGO walls while suppressing any competitive reaction involving interfering species which are rejected by the rGO walls. Figure 1

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